A gas thermometer of this kind is described in German Patent Disclosure Document 30 31 678, HIRT. It serves for the direct measurement of the temperature of hot fluids. To protect the sound generator and the sound receivers against high temperatures the latter are disposed at a comparatively great distance from the actual area of measurement, and to exclude the influence of the sliding temperature transition between the measuring range at high temperature and the sound-generator/sound-receiver range held at lower temperature the two reflector walls are provided, disposed in the measurement area at a distance apart in the direction of propagation of the sound. By measuring the phase shift of the portions of the sound field reflected by these two reflector walls a working signal is then obtained which, under otherwise unchanged measuring conditions, depends only on the temperature of the volume of gas situated between the reflector walls.
In the known measuring system this constitutes a nonreleasable component of the container in which the hot fluid that is to be measured is situated.